This invention relates to programmable logic array integrated circuits, and more particularly to programmable logic array integrated circuits having improved interconnections between the individual logic modules in the circuit.
Commonly assigned, co-pending patent application Ser. No. 754,017, filed Sep. 3, 1991 (hereby incorporated by reference herein) shows programmable logic array integrated circuits which are highly modular. In the circuits shown in that application, very similar types of interconnection pathways are used for all types of interconnections between the logic modules. This tends to simplify the design of the circuit. It also tends to simplify the software and/or other techniques used to program the circuit to implement particular logic functions. Thus the circuits shown in the above-mentioned application have several important advantages.
There is always room for further improvement, however, and there are some situations in which the provision of additional or alternative types of interconnections between the logic modules would have benefits sufficient to justify the additional circuit and programming complexity. Such additional interconnection paths may be desirable for making frequently needed interconnections, for speeding certain kinds of interconnections, for allowing short distance interconnections to be made directly without tying up more general purpose and therefore long distance interconnection resources, etc. There is also a continuing demand for logic devices with larger capacity. This produces a need to implement logic functions more efficiently and to make better use of the portion of the device which is devoted to interconnecting individual logic modules.
Wahlstrom U.S. Pat. No. 3,473,160 shows cellular logic arrays having programmable logic cells CA1, CA2, CA3, CB1, etc., which are interconnectable in a general way by data buses or lines X.sub.1B, X.sub.1C, Y.sub.12, and Y.sub.13, and which are also interconnectable in a more limited way by leads such as Z.sub.L, Z.sub.U, and Z.sub.B. Carter U.S. Pat. No. 4,642,487 shows the same kind of structure. However, in the Wahlstrom and Carter devices the special interconnection circuits merely duplicate connections which can be made through the general interconnect structure. The Wahlstrom and Carter special interconnection circuits may be faster than the general interconnect structure, and they may conserve general interconnect structure resources, but they do not add any capabilities to the circuit. For example, they do not facilitate the performance of more complex logic functions than can be performed by putting the same signals through the general interconnect structure rather than through the special interconnection circuits.
In view of the foregoing, it is an object of this invention to provide improved programmable logic array integrated circuits.
It is a more particular object of this invention to provide programmable logic array integrated circuits with additional possibilities for interconnections between the logic modules which are not mere duplications of interconnections which can be made through a general purpose interconnection network. SUMMARY OF THE INVENTION
These and other objects of the invention are accomplished in accordance with the principles of the invention by providing programmable logic array integrated circuits in which the outputs of at least some logic modules can be directly combined with the outputs of other logic modules without making use of the general interconnect network which is provided for such purposes as allowing the output of one logic module to be connected as an input to another logic module. In a preferred embodiment, each logic module includes an additional logic element for forming a logical combination of the normal output signal of that logic module and the output signal from another, preferably adjacent, logic module. The output signal from the other logic module is preferably applied directly to the additional logical element in the first logic module without going through the general interconnect circuitry. The output signal of the additional logic element in each logic module becomes the output signal of that logic module. Any number of logic modules can be interconnected in this way (e.g., in a series or cascade) to produce logic functions of any desired complexity. Any cascade interconnections which are not needed can be gated off.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.